This invention relates to a thin film magnetic head, particularly a thin film magnetic head of which the insulation layer is made of a polyimide resin.
Japanese Patent Laid-Open Nos. 135713/1977 and 93113/1981 proposed a thin film magnetic head of which the insulation layer was made of a polyimide resin obtained by heat curing a condensation or addition polymerization type polyimide precursor.
When a polyimide resin obtained by heat-curing a condensation or addition polymerization type polyimide precursor is used in the insulation layer of a thin film magnetic head, the insulation layer can be easily formed by a method comprising a step of coating and a step of heat curing, which method is superior in mass productivity to the formation of an inorganic insulation layer by vacuum deposition, sputtering deposition, etc. Further, the insulation layer thus obtained is superior in insulating properties and heat resistance to other organic insulation films.
However, the above-mentioned prior art had the following problems and, therefore, was unsatisfactory for practical use.
Specifically, since it is difficult for the condensation polymerization type polyimide precursor to melt during heat curing, unevennesses corresponding to the difference in levels due to the presence of a coil on a substrate are formed on the surface of the polyimide resin insulation layer. This in turn brings about the formation of an uneven magnetic film on the insulation layer, which makes it impossible to attain high permeability.
On the other hand, since the addition polymerization type polyimide precursor is poor in solubility in a solvent, an insoluble component is present in a solution of the precursor, which makes it difficult to form a uniform coating film. Therefore, this precursor is unsatisfactory for practical use, also.
Further, all the resins obtained by heat-curing the above-mentioned polyimide precursors are poor in adhesiveness, which leads to a problem with respect to reliability of the formed magnetic head.
Moreover, in the above-mentioned prior art, no sufficient consideration is given to the conditions under which an upper magnetic film is formed on the insulation layer. That is, the prior art also had a problem that no wide margin could be provided with respect to the temperature of formation of the upper magnetic film.
A permalloy (Ni-Fe alloy having high permeability exhibiting high permeability at a high-frequency region is generally used for the magnetic film of the thin film magnetic head. The permalloy film is formed by a plating, deposition or sputtering method. Among these methods, the deposition and sputtering methods are suitable for the formation of a magnetic film having excellent magnetic characteristics because the composition of the film to be formed can be easily controlled. However, in the deposition and sputtering methods, the substrate should be heated at a relatively high temperature, i.e. 280.degree. to 350.degree. C., in forming the permalloy film. For this reason, according to the studies conducted by the present inventors, the insulation layer on which the permalloy film is formed should have a high glass transition temperature. When the glass transition temperature is remarkably lower than the temperature of the substrate, the interlayer insulation layer exhibits rubber elasticity during the formation of the upper magnetic film. Therefore, the interlayer insulation layer undergoes deformation, such as buckling or protuberance of the edge portion of the film, due to the stress of the permalloy film serving as an upper magnetic film. All of these phenomena are factors causing the deterioration of the characteristics of the thin film magnetic head.
Further, the condensation polymerization type polyimide precursor forms water molecules when it undergoes a condensation reaction. The water molecules bring about defects, such as blisters, in the insulation layer, which often leads to the breaking or short-circuiting of a conductor provided in the thin film magnetic head.
As described above, the conventional thin film magnetic head in which the insulation layer is made of a polyimide resin is accomplished by the following serious problems: (i) insufficient flatness of the formed polyimide layer and (ii) occurrence of deformation during the formation of the upper magnetic film because of its low glass transition temperature (lower than 280.degree. C.). Further, it also has the following drawbacks: (iii) low adhesiveness and (iv) frequent occurrence of defects, such as blisters, due to the presence of water molecules.
An invention previously filed with the U.S. Patent and Trademark Office by some of the present inventors [filed on Feb. 18, 1986 with the U.S. Patent and Trademark Office under Application Ser. No. 829,824 which is now U.S. Pat. No. 4,686,147 (issue date: Aug. 11, 1987)]improved the flatness of the above item (i) but provided no improvement in the above item (ii) because the glass transition temperature of the resin used was lower than 280.degree. C. The above-described U.S. Patent corresponds to Japanese Patent Laid-Open Nos. 24408/1987 and 188712/1986.